Recently, magnetic heads for perpendicular magnetic recording have been proposed in order to increase recording density and capacity of a magnetic disk device, or to reduce the size thereof. In this magnetic head, a recording head includes a main magnetic pole for generating a perpendicular magnetic field, a write shield magnetic pole arranged on a trailing side of the main magnetic pole with a write gap interposed therebetween, and a coil that feeds magnetic flux to the main magnetic pole. Further, a high frequency assist head has been proposed, in which a high frequency oscillator, for example, a spin torque oscillator, is disposed between a medium-side end portion of the write shield magnetic pole that faces the main magnetic pole and the main magnetic pole. In this structure, a current is fed through the main magnetic pole and the write shield magnetic pole to the high frequency oscillator.
In the high frequency assist head, a high frequency magnetic field is applied to a recording medium through oscillation of the high frequency oscillator, which easily causes a magnetization reversal in a medium recording layer, hence to improve the recording ability. This oscillation of the high frequency oscillator may be achieved while recording when a weak magnetic field is applied from a recording gap of a magnetic core to a recording medium. However, when the magnetic field applied from a recording gap of a magnetic core to a recording medium is strong, a magnetic field within the recording gap can be too strong, making favorable oscillation of the high frequency oscillator difficult to achieve.